Laser light emitting device, laser beacon device and laser imager display device

ABSTRACT

A laser beam generating device which, by addition of a simplified structure, can enlarge the spectral width of the laser light and lower the coherence to a moderate value. The laser light generating device includes a first laser light source 31, a second laser light source 32, phase modulation units 34, 35 for phase-modulating the beams from the light sources with a sole frequency component or plural frequency components, and an sum frequency generating unit 33 for producing a light beam of the shorter wavelength based on the wavelength of the light phase-modulated by the phase modulation units 34, 35. The fundamental wavelength laser light generated by the first laser light source 31 is phase-modulated by the phase modulation unit 34 based on a pre-set modulation amplitude and the modulation frequency so as to be enlarged in spectral width before being incident on the sum frequency generating unit 33. The sum frequency generating unit 33 generates an sum frequency based on the laser light beams from the light sources for conversion to short wavelengths and for enlarging the spectral width. The result is that the distance of coherence of the outgoing laser light is shortened for removing the speckle noise.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to a laser light emitting device operating as alight source for the laser light which for which a moderately widespectral width is required.

Heretofore, attempts have been made for utilizing a high output laser ina variety of industrial fields by exploiting monochromaticity (narrowspectral band) of the laser light. In particular, the laser lightexcited by a laser light source employing neodymium aluminum garnet(Nd:YAG) oscillating at a longitudinal single mode by a Q-switchingmethod, referred to hereinafter simply as a Nd:YAG switch laser, and thelaser light obtained on wavelength conversion of this laser light,exhibit high peak intensity, so that such laser light is expected to beused in a number of industrial fields.

Such Q-switch laser oscillates in general in longitudinal multiple modeoscillation. If the line width (spectra width) of the frequencycomponents is broader, the problem of chromatic aberration is raised.Although it is attempted to excite the laser light in a sole wavelengthusing an injection seed technique, the oscillation spectral widthbecomes excessively narrow to produce inconveniences in utilization.

The laser light having high monochromaticity, that is with a narrowspectral width, has high coherence and is susceptible to noise producedby the interference pattern (speckle noise) caused by interference ofthe laser light itself with irregular phase relation with diffusedlight, such as stray light having different propagation distances.Conversely, the laser light with low monochromaticity, exhibits lowcoherence, however, it has a broad spectral width and is susceptible tochromatic aberration.

The laser light is used in, for example, a laser beacon device. Thislaser beacon device is investigated as means for improving resolution ofan optical system employed in observation of heavenly bodies or inintersatellite light communication, as described in "Laser BeaconAdaptive Optics", Physics news, pp.14 to 19, Jun. 1993.

The laser beacon device radiates a laser light into air and emits lightfrom sodium atoms in atmosphere by resonant absorption. The laser beacondevice operates for detecting atmospheric disturbances by detecting thelight emitted by the sodium atoms on the ground surface, while operatingfor correcting the atmospheric disturbances using an adaptive opticalsystem for improving resolution of a telescope.

The sodium atoms emit light on resonant absorption of the laser light ofa wavelength in the vicinity of 589 nm. For realizing high efficiencyresonant excitation of sodium atoms, a high output laser light source,correctly coincident with the absorption spectrum of sodium atoms inboth the frequency and frequency width, is required.

As a high-output light source having a wavelength of 589 nm, there hasso far been proposed a laser light source comprised of a first laserlight source 101 having a Nd:YAGQ switch laser generating a fundamentalwavelength laser light of a wavelength of 1319 nm narrowed in frequencyusing an injection seed technique, a second laser light source 102having a Nd:YAGQ switch laser generating a fundamental wavelength laserlight of a wavelength of 1064 nm similarly narrowed in frequency and anLBO crystal 103 for generating an sum frequency laser light from thefundamental wavelength laser light with the wavelength of 1319 nm andthe fundamental wavelength laser light with the wavelength of 1064 nm.

The injection seed technique, used in the first laser light source 101and in the second laser light source 102, excites the laser light byoscillation in a sole wavelength for correctly coinciding the wavelengthof the laser light after additive frequency generation with theabsorption wavelength of sodium atoms.

The absorption frequency width of sodium atoms, subjected to the Dopplereffect, is on the order of 3 GHz. Since the laser light with thewavelength of 589 nm, obtained by a high output light source shown inFIG. 1, is narrowed in frequency by injection seed in each light source,the laser light obtained by the high output light source exhibits highfrequency stability. The spectral width of the basic wavelength laserlight, obtained by the high output laser light source, is narrowed up toapproximately 25 MHz, which is the line width of a transform limitedpulse, and which is only 1/120 of the absorption frequency width ofsodium atoms. Therefore, the resonance efficiency between the laserlight and the sodium atoms is low.

The laser light is also applied to, for example, a laser image displaydevice.

In the above laser image display device, if a laser light beam with anarrow spectral width is used, the speckle noise tends to be producedbecause of the high coherence of such laser light. Such speckle noisegenerates granular speckles in the laser image display device, thussignificantly deteriorating the picture quality.

Thus, in the application of the laser light, it is crucial that thespectral width of the laser light be controlled to a moderate value forobviating problems produced in connection with chromatic aberration andspeckled noise.

Several methods for broadening the spectral width of the laser light hasso far been proposed. For example, it may be envisaged to oscillate thelaser light in longitudinal multiple mode or to use the laser lightoscillated in longitudinal multiple mode from the outset. In this case,the spectral width tends to be broadened excessively to raise theproblem in connection with chromatic aberration. In addition, thestructure of the laser light generating device itself needs to bechanged, thus lowering the light emitting efficiency. That is, it hasbeen difficult to increase the spectral width moderately to a desiredvalue.

Furthermore, the laser light beam obtained on wavelength conversion onadditive frequency mixing of two or more sorts of laser light beamsoscillated in longitudinal multiple modes is unstable in intensity.

For removing the speckle noise, attempts have been made in improving thelaser light projection system. For example, it has been proposed in JPPatent Kokai Publication JP-A-55-65940 (1980) that, in a laser imagedisplay device, a screen or a laser light source is oscillatedmechanically. This, however, is infeasible if the screen size is larger.

Thus it is difficult to control the spectral width of the laser light toa desirable value to remove the speckled noise.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a laserlight generating device in which the spectral width of the laser lightis broadened by addition of a simpler structure for deteriorating thecoherence to a moderate value.

It is another object of the present invention to provide a laser beacondevice and a laser image display device employing the above-mentionedlaser light generating device.

According to the present invention, there is provided a laser beamgenerating device having a laser light source, phase modulation meansfor phase modulation of a laser beam radiated from the laser lightsource with a sole frequency component or a plurality of frequencycomponents, and wavelength conversion means for converting thewavelength of the laser light phase-modulated by the phase modulationmeans into other wavelengths.

With the present laser beam generating device, the fundamentalwavelength laser beam, generated by a laser light source, isphase-modulated by phase modulation means with a pre-set modulationamplitude and a pre-set modulation frequency so as to be enlarged inspectral width. The phase-modulated laser beam is then converted bywavelength conversion means into a laser beam of a shorter wavelength atthe same time as it is further enlarged in spectral width. The result isthat the coherence distance of the laser beam is shortened to suppressthe speckle noise.

By carrying out phase modulation with plural frequencies, a laser beamhaving a continuous spectrum can be generated. With such laser beam,coherence is sufficiently reduced, while generation of the speckle noiseis suppressed. The laser beam radiated from the wavelength conversionmeans has its coherence controlled by the modulation amplitude and themodulation frequency used in the phase modulation means.

With the above-described laser beam generating device of the presentinvention, it becomes possible to realize coherence required of theoptical device to diminish the speckle noise by addition to the lightsource of an optical system of a simplified structure configured forcontrolling the modulation frequency and the modulation frequency at thetime of phase-modulating the fundamental wavelength laser beam.

In addition, in carrying out phase modulation using plural modulationfrequencies, it becomes possible to generate a continuous spectrum tofurther reduce the speckle noise.

Moreover, if phase modulation is performed prior to wavelengthconversion, the electro-optical crystal can be prevented from beingdamaged by the laser beam since the laser beam incident on theelectro-optical crystal is a near-infrared light which is moretransparent to electro-optical crystals than shorter wavelength light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the structure of a conventional laser light generatingdevice.

FIG. 2 shows an illustrative structure of a laser light generatingdevice of the present invention used as a light source for a laserbeacon device.

FIGS. 3A, 3B and 3C are graphs showing phase modulation performed by thelaser generating device.

FIG. 4 shows an illustrative structure of a phase modulation unit in thelaser modulation unit.

FIG. 5 shows an illustrative structure of the laser light generatingdevice used as a light source for the laser image display device.

FIG. 6 shows laser light spectra and coherence curves.

FIGS. 7A and 7B are graphs showing coherence in a length exceeding thecoherent length of a laser light beam phase-modulated by the phasemodulation unit.

FIG. 8 shows a second illustrative structure of a phase modulation unitin the laser modulation unit.

FIG. 9 shows a third illustrative structure of a phase modulation unitin the laser modulation unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, preferred embodiments of the laser lightgenerating device according to the present invention will be explainedin detail.

FIG. 2 shows a laser light generating device for generating the laserlight of 589 nm used in, for example, a laser beacon device.

FIG. 2 shows a laser light generating device for generating the laserlight of 589 nm used for example in a laser beacon device.

The laser light generating device is made up of light sources 31, 32 forgenerating fundamental wavelength laser beams, phase modulation units34, 35 for phase modulating the fundamental wavelength laser light beamsfrom the light sources 31, 32 with one or plural frequency components,and an sum frequency generating unit 33 for converting the wavelength ofthe light beams phase-modulated by the phase modulation units 34, 35into other wavelengths.

Each of the phase modulation units 34, 35 has an electro-opticalelement, while the sum frequency generating unit 33 has a non-linearoptical crystal element.

Referring to FIG. 2, the fundamental wavelength laser light beam of awavelength of 1319 nm, radiated by the first laser light source 31, isincident on the phase modulation unit 34 for phase modulation. Thefundamental wavelength laser light, with a wavelength of 1064 nm,radiated from the second laser light source, is incident on the phasemodulation unit 35 for phase modulation. The fundamental wavelengthlaser light beam prior to wavelength conversion is phase-modulated bythe phase modulation units 34, 35 with one or plural modulationfrequencies so that the respective fundamental wavelength laser beamsare increased in spectral widths. The phase-modulated basic wavelengthlaser light beams are incident on the sum frequency generating unit 33.The sum frequency generating unit 33 performs sum frequency mixing, thatis wavelength conversion, using the fundamental wavelength laser lightbeams of 1319 nm and 1064 nm. The laser light of 589 nm, after thewavelength conversion, is increased in spectral width than thefundamental wavelength laser beams. The laser beam with the wavelengthof 589 nm, obtained after sum frequency mixing or wavelength conversion,is sufficiently widened in the spectral width, so that the resonanceefficiency can be improved if the laser beam is used for resonanceexcitation of sodium atoms.

The principle of phase modulation in the electro-optical element andchanges in the power spectrum produced by phase modulation will now beexplained.

FIG. 3A shows the power spectrum in an initial stage of the fundamentalwavelength laser beam.

If a signal voltage of a periodic function φ(t) is impressed across anelectro-optical modulation device, provided with an electro-opticalelement, an electrical field E(t) of the laser light undergoes phasemodulation as indicated by the equation (1):

    E(t)=E.sub.0 (t)exp  i2πf.sub.0 t+iΦ(t)!            (1)

    Φ(t)∝φ(t)                                   (2)

where f₀ is the center frequency of the pre-modulation laser beam.

In the above equations, Φ(t) is a phase modulation function andproportionate to the signal φ(t) impressed across the electro-opticalmodulation device. This is shown in the equation (2). In particular, ifΦ(t) is a sine wave having an amplitude m and a frequency f_(m) asindicated by the equation (3), the electrical field E(t) may beexpanded, using the Bessel function series Jk(m), as indicated by theequation (4):

    Φ(t)=m sin2πf.sub.m t                               (3) ##EQU1##

At this time, the power spectrum becomes the sum total of the spectralcomponents of the intensities Jk² (m) and the frequencies (f₀ +kf_(m)),where k is an integer, as indicated in FIG. 3B. The spectral intensityof the center frequency f₀ is decreased by phase modulation and becomesequal to {J₀ (m)}² times that prior to modulation.

If phase modulation is done a plural frequencies, the phase modulationfunction Φ(t) is as indicated by the equation (5): ##EQU2## such thatphase modulation in each frequency operates additively andmultiplicatively as to the frequency and intensity, respectively.

For example, a spectrum composed of the intensity Jk² (m1) and thefrequency (f₀ +kf_(m1)), produced by phase modulation at a modulationamplitude m1 and the modulation frequency f_(m1), is splitted by phasemodulation at another modulation frequency f_(m2) into spectralcomponents having the intensities of Jk² (m1)·Jl² (m2) and thefrequencies (f₀ +kf_(m1) +1fm₂), where k and 1 are integers, asindicated in FIG. 3C, and thus becomes an aggregate of a large number ofspectra.

It may be seen that, by effecting a plurality of, for example, two,phase modulation operations at respective different modulationfrequencies, the initial power spectrum may be separated into a largenumber of spectra, with the spectral distances becoming denser. Thespectral intervals become smaller than the smallest one of the pluralmodulation frequencies. If the least common multiple of the modulationfrequencies is a sufficiently large value, the number of the spectralcomponents becomes substantially equal to (2m₁ +1)×(2m₂ +1).

If the first phase modulation is carried out with a high frequencyvoltage signal of a voltage amplitude such that the frequency is fm1=350MHz and the phase modulation amplitude m1=2 radians (phase modulationfunction Φ(t)) and the second phase modulation is carried out with ahigh frequency voltage signal of a voltage amplitude such that thefrequency is fm2=100 MHz and the phase modulation amplitude m2=2radians, the spectral interval becomes equal to 100 MHz, with the numberof the spectral components becoming (2×2+1)×(2×2+1)=25.

In the phase modulation, described above, it is not the spectral linewidth of the fundamental wavelength laser light beam but the width of alarger number of the spectral components into which the fundamentalwavelength laser light beam is splitted, that is broadened. The width ofan envelope of the aggregate of the spectral components thus separatedis defined herein as the entire spectral width. That is, the coherentlength is shortened by the spreading apart of the entire spectrum, thusreducing the speckle noise.

Referring to FIG. 2, the first laser light source 31 is an Nd:YAGQswitch laser, narrowed in frequency from the laser beam having thewavelength of 1319 nm by injection seeding. On the other hand, thesecond laser light source 32 is an Nd:YAGQ switch laser, narrowed infrequency from the laser beam having the wavelength of 1064 nm byinjection seeding.

High-frequency generating units 36, 37 generate, as high-frequencysignals, a signal voltage of a periodic function φ(t) for obtaining theabove-mentioned phase modulation function Φ(t), such as a sine wavevoltage signal composed of one or plural frequency components, by thephase modulation units 34, 35, respectively, and output the resultinghigh-frequency signals to amplifiers 38, 39, respectively. Theamplifiers 38, 39 amplify the input high-frequency signals and outputthe resulting amplified high-frequency signal to the phase modulationunits 34, 35, respectively.

The phase modulation unit 34 is arranged as shown in FIG. 4, usingplural potassium titanyl phosphate phosphate crystals (KTiOPO₄ :KTP) aselectro-optical crystals, as shown in FIG. 4. Two KTP crystals 41a, 41bare mounted on a mount 42. One of the electrodes of each of KTP crystals41a, 41b is connected via a connector 43 to the amplifier 38, with theother electrode being grounded. This impresses the high-frequencyvoltage from the high-frequency signal generating unit 36 across theelectrodes of the KTPs 41a, 41b.

The phase modulation unit 34 also phase-modulates the incidentfundamental wavelength laser beam with the wavelength of 1319 nm with aphase modulation function obtained on the basis of the sine wave voltagesignal from the high-frequency signal generating unit 36, and spreadsthe entire spectral width of the fundamental wavelength laser beam withthe wavelength of 1319 nm to Δf₁ (Δf₁ ≈2×fm₁ ×m₁). That is, theelectrical field E₁ (t) of the fundamental wavelength laser beam at thistime is of a value represented by the equation (6):

    E.sub.1 (t)=E.sub.01 (t)exp  i2πf.sub.1 t+im.sub.1 sin2πf.sub.m1 t!(6)

Similarly, the phase modulation unit 35 also phase-modulates theincident fundamental wavelength laser beam with the wavelength of 1064nm with a phase modulation function obtained on the basis of the sinewave voltage signal from the high-frequency signal generating unit 37,and spreads the entire spectral width of the fundamental wavelengthlaser beam with the wavelength of 1064 nm to Δf₂ (Δf₂ ≈2×fm₂ ×m₂). Thatis, the electrical field E₂ (t) of the fundamental wavelength laser beamat this time is of a value represented by the equation (7):

    E.sub.2 (t)=E.sub.02 (t)exp  i2πf.sub.2 t+im.sub.2 sin2πf.sub.m2 t!(7)

The sum frequency generating unit 33 has, for example, boron oxidelithium (LiB₃ O₅ :LBO) crystals, as non-linear optical crystal elements.The sum frequency generating unit 33 generates and radiates the laserbeam having the wavelength of 589 nm in accordance with the principle ofsum frequency generation, as later described, using the phase-modulatedfundamental wavelength laser beams with the wavelengths of 1319 nm and1064 nm. The entre spectral width Δf₃ of the laser beam,wavelength-converted into the wavelength of 589 nm, is of the same orderof magnitude as the sum of the entire spectral widths Δf₁ and Δf₂ of therespective fundamental wavelength laser beams prior to wavelengthconversion, that is, Δf₃ =Δf₁ +Δf₂. Since the allowable wavelengthwidths of the respective laser beams used for this wavelength conversionare of sufficient widths, the entire spectral width is increased, sothat the wavelength conversion efficiency is not lowered. An electricalfield E₃ (t) of the laser beam, obtained on wavelength conversion, is ofa value represented by the equation (8): ##EQU3##

The principle of sum frequency generation is now explained. In anon-linear optical crystal element, non-linear polarization notproportionate to the magnitude of the external electrical field appliedfrom outside is generated. If, of the non-linear polarization, thenon-linear second-order susceptibility is not zero, and two light beamswith frequencies of v1, v2 are incident, a non-linear polarization withthe frequency of v3, where v3=v1+v2, is induced in the crystal. That is,if the laser beams with the wavelengths of λ1 and λ2 are incident, thewavelength λ3 of the light, generated on frequency addition and radiatedfrom the non-linear optical crystal, satisfies the following relation(9): ##EQU4##

That is, the wavelength of the laser beam, produced by frequencyaddition of a laser beam with a wavelength of 1319 nm and a laser beamwith a wavelength of 1064 nm, is 1/(1/1319+1/1064)=589 nm.

It is assumed that phase modulation is carried out using a sine wave ofa sole frequency component as a phase modulation function. If thefundamental wavelength laser beam with a wavelength of 1319 nm isphase-modulated, with f_(m1) of 350 MHz and m1 of 2 radians, Δf1 becomesequal to 2×350 MHz×2=1.4 GHz. That is, the spectral width of thefundamental wavelength laser beam with the wavelength of 1319 nm in itsentirety has been enlarged to approximately 1.4 GHz.

On the other hand, if the basic wavelength laser beam with a wavelengthof 1064 nm is phase-modulated, with f_(m2) of 350 MHz and m2 of 2radians, Δf2 becomes equal to 2×350 MHz×2=1.4 GHz. That is, the spectralwidth of the fundamental wavelength laser beam with the wavelength of1064 nm in its entirety has been enlarged to approximately 1.4 GHz.

The spectral width Δf3 of the laser beam obtained on frequency additionis the sum of Δf1 and Δf2, and hence becomes substantially equal to 1.4GHz+1.4 GHz=2.8 GHz, thus substantially coinciding with the absorptionline width of sodium atoms subjected to the Doppler effect, for therebyassuring efficient excitation of sodium atoms.

In the above laser light generating device, the fundamental wavelengthlaser beam prior to wavelength conversion is phase-modulated andsubsequently the laser beam of the sum frequency is generated foradditively demonstrating the respective phase modulation effects. Itsuffices if the spectral width of the laser beam prior to wavelengthconversion is one-half the spectral width desired of thepost-wavelength-conversion laser beam.

Meanwhile, if the phase conversion is carried out using a solemodulation frequency fm, the power spectrum is an aggregate of spectralcomponents having frequency intervals equal to fm. Thus the frequencyinterval of the spectrum of the laser beam obtained on phase modulationof two laser beams of two different wavelengths using the modulationfrequency of the same order of magnitude followed by sum frequencygeneration is on the order of fm.

In the above embodiment, no more than about eight spectral componentsare present within the spectral width of 2.8 GHz, with the spectralinterval being 350 MHz. On the other hand, for exciting sodium atomsefficiently, it is desirable to generate a continuous spectrum with aspectral width on the order of 3 GHz.

For producing such spectrum by phase modulation, it is necessary toreduce the frequency interval to generate a large number of spectralcomponents. On the other hand, since the spectral width of the laserbeam radiated from the laser light source is on the order of 25 MHz, acontinuous spectrum may be produced if the frequency interval is set toabout 25 MHz. For realizing a continuous spectrum with a sole modulationfrequency, the modulation frequency needs to be reduced to a smallervalue of 25 MHz. For providing a spectral width of 2.8 GHz, it sufficesif the modulation amplitude is set to about 28, the spectral width ofthe laser beam of each wavelength is set to 2×28×25 MHz≈1.4 GHz and thespectral width is further doubled through the sum frequency generatingprocess. However, such a larger modulation amplitude in effect cannot beachieved.

By carrying out plural phase modulation operations using lowermodulation frequency components, it becomes possible to realize aspectrum of a smaller frequency interval than is possible with phasemodulation by a sole frequency component. Specifically, each of thefundamental wavelength laser beams of two sorts of the wavelengths isphase-modulated with the modulation frequency of 350 MHz and themodulation amplitude of 2 radians. In addition, one or both of thefundamental wavelength laser beams prior to wavelength modulation isphase-modulated with the modulation frequency of 100 MHz and themodulation amplitude of 2 radians for separation into dense spectralcomponents with the frequency interval of 100 MHz. Further, one or bothof the fundamental wavelength laser beams prior to wavelength modulationis phase-modulated with the modulation frequency of 25 MHz and themodulation amplitude of 2 radians.

The spectrum of the laser beam resulting from the above-described phasemodulation is an aggregate of a large number of spectral components withthe frequency intervals of 25 MHz, thus achieving a substantiallycontinuous spectrum. Thus, by phase modulation employing pluralfrequencies, it becomes possible to diminish the frequency intervalwithout the necessity of increasing the modulation amplitude, as aresult of which a continuous spectrum can be generated for improving theexcitation efficiency of, for example, sodium atoms.

Among the methods for doing phase modulation using plural frequencycomponents, there are a method consisting in arraying pluralelectro-optical crystals in series and impressing voltages of differentfrequencies across these crystals, and a method consisting in impressingsignals of plural frequency components across a sole electro-opticalcrystal. In particular, with the former method, the driving voltage ofthe phase modulation unit can be lowered by arranging circuits that maybe in electrical resonance at a frequency applied to eachelectro-optical crystal, as will be explained subsequently. With thelatter method, the number of the electro-optical crystals can be reducedthus assuring a low production cost.

Such phase modulation at plural frequencies is effective in removing thespeckle noise, as will, be explained subsequently.

As the electro-optical crystals, use may be made of all electro-opticalcrystals capable of transmitting the near-infrared light, in addition toKTP. In particular, MTiOXO₄ (M=K, Rb, Tl, NH₄, Cs, X=P, As), which is aderivative of KTP, exhibits superior electro-optical effects, and isless prone to damages otherwise caused by a higher output laser beam, sothat it is effective as a high-output laser.

In the above illustrative embodiment, the fundamental wavelength laserbeam having the wavelength of 1319 nm and the fundamental wavelengthlaser beam having the wavelength of 1064 nm can be combined together sothat the two fundamental wavelength laser beams will be phase-modulatedsimultaneously by one and the same phase modulation unit. Although themodulation frequencies and modulation amplitudes of the phase modulationperformed on the respective fundamental wavelength laser beams cannot beset independently, the spectral width can be effectively widened byselecting the suitable frequency and suitable modulation amplitude.

The favorable effect of the present invention can be achieved byemploying the modulation frequency and the modulation amplitude otherthan those given in the present illustrative embodiment. While theabsorption line width of sodium atoms, subjected to the Doppler effect,is as broad as 3 GHz, the frequency width of the frequency-stabilizedgeneric solid Q-switch laser is as small as tens of MHz. The resonanceexcitation efficiency can be sufficiently improved by increasing thefrequency width of the laser beam to approximately 500 MHz instead of toas high as 3 GHz. Although a continuous shape of the spectrum isdesirable, the excitation efficiency can be improved sufficiently byemploying a laser beam made up of four or more spectral components eachhaving a spectral intensity not higher than 30% of the sum of the entirespectral intensity.

FIG. 5 shows an illustrative configuration of a laser beam generatingdevice of the present invention as applied to a laser image displaydevice.

In general, an image display device is in need of light sources of threecolors, namely red, green and blue colors. Thus the above laser imagedisplay device employs, as a light source, a first laser light source 51having a Nd:YAGQ switch laser generating a laser beam with a wavelengthof 1319 nm, a second laser light source 52 having a Nd:YAGQ switch lasergenerating a laser beam with a wavelength of 1064 nm and a third laserlight source 53 having a Nd:YAGQ switch laser generating a laser beamwith a wavelength of 946 nm. Specifically, the second harmonics of thebasic wavelength laser beam, radiated by the first laser light source51, is the red-hued laser beam, while the second harmonics of the basicwavelength laser beam, radiated by the second laser light source 52, isthe green-hued laser beam, and the second harmonics of the basicwavelength laser beam, radiated by the third laser light source 53, isthe blue-hued laser beam.

The fundamental wavelength laser beams, radiated by the respective laserlight sources, are phase-modulated with a sole frequency or pluralfrequencies by phase modulation units 54, and wavelength-converted bysecond harmonics generating units 55 to form second harmonics forwidening the spectral width. That is, temporal coherence is lowered. Thegenerated second harmonics are then modulated by image signals by anintensity modulation unit 56. Subsequently, the lowering in temporalcoherence is converted into that in spatial coherence for reducing thespeckle noise in the laser beam. In general, a screen 511 is moved forreducing the speckle noise. This, however, is unnecessary with thepresent embodiment.

The phase modulation unit 54 is provided independently for each of thelaser light sources, and includes an electro-optical crystal, such asKTP. The phase modulation unit 54 phase-modulates the incident basicwavelength laser beam by a pre-set modulation frequency and a pre-setmodulation amplitude, impressed from outside, for widening the spectralwidth, as described previously. Any optional coherence may be achievedby suitably selecting the modulation frequency and the modulationamplitude.

Similarly to the phase modulation unit 54, the second harmonicsgenerating unit 55 is provided independently for each laser lightsource, and includes a non-linear optical crystal, such as LBO or BBO.The second harmonics generation unit converts the phase-convertedfundamental wavelength laser beam into second harmonics, that is halvesthe wavelength of the fundamental wavelength laser beam. If the spectralwidth of the fundamental wavelength laser beam is enlarged by phasemodulation to, for example, 500 MHz, the spectral width of the laserlight subsequent to wavelength conversion is twice that of thefundamental wave, or 1 GHz, with the coherent length being on the orderof 0.2 m.

A decoherer 57, provided independently for each laser light source,splits the laser light, affords the optical path length difference ofnot less than the coherent length to the split laser beams andsubsequently synthesizes the split laser beams.

A polygonal mirror 58 is a rotating member having a series of planarreflective surfaces on its peripheral surface, and constitutes, alongwith a galvanomirror 59 as later explained, a deflection optical systemfor sweeping the light from the respective laser light sources on ascreen 511. The polygonal mirror 58 also reflects the laser lightoutgoing from the decoherers 57 and combined together in order to causethe reflected laser light to fall on the galvanomirror 59 via aprojection lens 510.

The galvanomirror 59 is supported for rotation by the galvanomotor 512and is rotated at an elevated speed in association with the laser lightsequentially sent in association with the rotation of the polygonalmirror 58 for reflecting and projecting the incident laser light on thescreen 511.

For reducing the speckle noise, it is necessary for the coherence of thelaser beam to become sufficiently small in the presence of an opticalpath length difference not less than the coherent length. To this end,it is desirable that the spectrum of the post-wavelength-conversionlaser light beam be made up of not less than at least four spectrallines each having non-negligible intensity and that the spectralintensity of each spectral component be not more than 30% of the entirespectral intensity, as now explained.

If the spectrum of the laser beam subsequent to wavelength conversion issplit by phase modulation, but the number of spectral components is notless than, for example, three, or if the spectral intensity of one ofthe spectral components is as large as not less than 30% of the entirespectral intensity, the spectral intensity is concentrated in aparticular frequency, so that the laser light cannot be said to be ofmultiple modes, such that coherence cannot be decreased sufficiently.This is not favorable in view of decoherer designing.

Conversely, if the spectral intensity of each spectral component is notmore than 30% of the entire spectral intensity, and there exist four ormore spectral components, the spectrum of the laser beam is split into alarger number of spectral components with good equilibrium, so thatcoherence is effectively decreased. The length at which coherence isagain increased is relatively spaced apart as compared to the coherentlength. This is favorable for the designing of decoherers.

Reference is had to FIG. 6 showing the spectrum of the laser light andcoherence curves in case the laser beam is phase-modulated with ahigh-frequency voltage signal of a certain frequency to a longitudinalsingle mode laser. The phase modulation amplitude m is varied in a rangeof from 1.2<m<2.4. If the phase modulation is performed with a solefrequency and with a smaller modulation amplitude, as in the presentembodiment, coherence becomes zero at a certain optical path lengthdifference (coherent length), however, it is again increased withincreased optical path length difference. The reason is that, since theinterval between the spectral components is equal, coherence of theneighboring spectral components becomes strongly apparent.

In particular, if the modulation amplitude is small, such that m=1.2 to1.4, coherence again becomes maximum for the optical path lengthdifference of the order of 2.5 times the coherent length. In the case ofm=1.4, coherence again becomes maximum for the optical path lengthdifference of 200 cm for the coherent length of 80 cm. With a lightsource in which coherence again becomes maximum for the optical pathlength difference as small as about 2.5 times the coherent length, it isdifficult to remove the speckle noise using a decoherer.

If the modulation amplitude m is such that m=1.6 or higher, the spectrumis made up of four or more spectral components of non-negligibleintensities, with the intensities of the respective spectral componentsbeing not higher than 30% of the entire spectral intensity. In thepresent embodiment, the optical path length difference for whichcoherence again becomes maximum is not less than thrice the coherentlength. It should be noted that, if m=1.6, coherence again becomesmaximum with the optical path length of 200 cm for the coherent lengthof 60 cm. If the optical path length difference for which coherenceagain becomes maximum is sufficiently long as compared to the coherentlength, the speckle noise can be removed sufficiently by suitablydesigning the decoherers. The coherence again becoming maximum for theoptical path length difference about three times as long as the coherentlength cannot be said to be sufficient, however, it is effective todisplay the effect of removal of the speckle noise.

The entire spectral width of the laser beam wavelength-converted afterphase modulation is desirably not less than 500 MHz. The reason is that,in view of decoherer designing, the decoherer size tends to becomeexcessive for too long a coherent length. If the spectral width is 500MHz or higher, the coherent length is on the order of 0.4 m, so that thedecoherer can be designed with a practical size.

If phase modulation is performed with plural frequencies, moreoutstanding results can be achieved. If phase modulation is performedwith a sole frequency, there necessarily exists such an optical pathlength difference for which coherence again is increased for an opticalpath length difference longer than the coherent length, since thefrequency interval of each spectral component of the laser beam isequal. This state is shown in FIG. 7A. Conversely, if the phasemodulation is performed with plural frequencies, the intervals betweenthe spectral components are not the same, so that coherence is notincreased again with the optical path length difference exceeding thecoherent length. This state, shown in FIG. 7B, is favorable in removingthe speckle noise.

Although an embodiment of the present invention applied to a laser imagedisplay device has been described above, the laser beam is not limitedto the above-described embodiments, but may be modified in many ways.

For example, although the embodiment has been described above in whichthe Q-switch laser is used as a light source and an Nd:YAG as a solidlaser device is used as an illustrative example of the Q-switch laser, agas laser, such as krypton gas laser, or a semiconductor laser, such asa laser diode, may be used as a red light source.

Although the spectral width is enlarged in the above embodiment byexploiting the waveform conversion process, it is also possible todirectly perform phase modulation on the light source subsequent to thewaveform conversion. The low coherence and reduction of the specklenoise, characteristic of the present invention, may also be achieved inthe case of the laser light source not having the wavelength conversionprocess.

If the krypton gas laser or the laser gas diode is used as the red-huedlight source, and wavelength conversion is not performed, the laserlight from the light source is shorter in wavelength than thenear-infrared laser light, significant modulation effects may berealized at a low voltage at the time of phase modulation.

Although the laser beams from the three light sources of red, green andblue colors are phase-modulated independently of one another, it is alsopossible to combine the three laser beams together and to perform phasemodulation on the combined laser beam by a sole phase modulation unitfor three colors simultaneously. In this case, the laser beam afterphase modulation is separated by a color separation filter before beingincident on a second harmonics generating unit as a wavelengthconversion unit.

Since the spectral width can be controlled by controlling the modulationamplitude or modulation frequency at the time of phase modulation, itbecomes possible to realize the laser light beam of which the specklenoise and the chromatic aberration have reached the practical level.

Although the present invention is applied to an illustrative embodimentin which the laser light emitting device is used as a light source forthe laser beacon device and the laser image display device, it may alsobe used as a light source for other optical devices.

In addition, although a light source oscillating with a longitudinalsingle mode is used as a light source for the fundamental wavelengthlaser light subjected to phase modulation, it is also possible to use alaser light source of the longitudinal multiple mode. For example, if aNd:YAGQ switch laser is oscillated in multiple modes with the frequencyinterval on the order of 400 MHz, this laser beam may be phase-modulatedwith the modulation frequency not higher than 400 MHz, such as 100 MHz,for approaching the power spectrum of the laser light to as close to acontinuous spectrum as possible for diminishing the coherencysufficiently.

Moreover, although the laser light source oscillated with a Q-switchoscillation is used as a light source of the fundamental wavelengthlaser light to be phase-modulated, a continuously oscillated laser lightsource may also be employed. In this case, thepost-wavelength-conversion laser light is phase-modulated since if theresonant frequency of a resonator inclusive of a non-linear opticalcrystal is coincident with the frequency of the fundamental wavelengthlaser light beam, wavelength conversion is performed with a highconversion efficiency, whereas, if the spectrum is enlarged beforewavelength conversion, the wavelength conversion efficiency is loweredsignificantly.

An illustrative structure of the phase modulation unit, in which twoKTPs are arrayed in series in the longitudinal direction, as shown inFIG. 4, it is possible to use such a structure in which a KTP 62 whichis 30 mm long along the lengthwise direction and 2 mm long along otherdirections is used, an electrode 63 is formed on a lateral side thereof,a groove electrode 64 is formed at a distance of, for example, 0.5 mminwardly from the electrode 63 at a distance s from the electrode 63,with the electrode-to-electrode distance being s, with the electrode 63being grounded and the groove electrode 64 being connected to theamplifier 38, as shown in FIG. 8. In such case, the laser light beam tobe phase-modulated is caused to be incident to an area between the twoelectrodes. By this structure, the input voltage to the KTP 62 from thehigh frequency signal generator 36 may be diminished. The phasemodulation unit shown in FIG. 8 is particularly effective if no damageis done to the crystal due to the laser beam even if the input laserbeam is sufficiently reduced in beam diameter.

Another structure of reducing the input voltage to the KTP is shown inFIG. 9, in which two KTPs 62a, 62b are arranged in series in thelengthwise direction on distinct mounts 42a, 42b, respectively, anelectrode 65 is provided on a surface of the KTP 62a opposite to thesurface thereof facing the mount 42a, and in which a coil 63 isconnected between the electrode 65 and an amplifier 38a.

If, with the electrical capacitance of the electro-optical crystal beingC_(xtal) and with the inductance of a coil having its end connected toan electrode arranged on the electra-optical crystal being L, ahigh-frequency signal of the frequency fr is supplied to the oppositeend of the coil, the electro-optical crystal and the coil constitute anelectrical resonation system as represented by the equation (10):##EQU5##

To each phase modulation unit, a signal of a single frequency may beimpressed by the first high frequency signal generating unit 36a or thesecond high frequency signal generating unit 36b for setting the coilinductance for enabling electrical resonation responsive to eachfrequency in accordance with the equation (10) across the KTPs 62a, 62bfor impressing the voltage on the order of one-tenth for achievingmodulation amplitude of the equivalent size. As the electro-opticalcrystal, those having a low dielectric loss, that is high electricalresistance, such as dihydrogen potassium phosphate (KH₂ PO₄), dihydrogenammonium phosphate (NH₄ H₂ PO₄), β-barium borate (β-BaB₂ O₄, β-BBO),rubidium titanyl arsenate (RbTiOAsO₄), cesium titanyl arsenate(CsTiOAsO₁₄), lithium niobate (LiNbO₃) or lithium tantalate (LiTaO₃),may be effectively employed.

If, with a view to enlarging the spectral width with a high frequencyand a small modulation amplitude, phase modulation is to be performedwith a signal of a higher frequency, such as 400 MHz or higher, thephase modulation unit may be configured so that an electro-opticalcrystal will be arranged within the inside of a microwave waveguide. Insuch case, a smaller impressed voltage suffices if the microwavewaveguide is oscillated in resonation at the time of impressing anelectrical voltage across the electro-optical element.

It is also possible to provide a pair of mirrors on both sides of theelectro-optical crystal, in which case the laser light beam incident onthe crystal is caused to travel repeatedly through the inside of thecrystal for protracting the length along which phase modulation occursfor reducing the required voltage. Meanwhile, this mirror may beprovided on the outside of the crystal. Alternatively, a high reflectivefilm may be deposited on the crystal surface on a portion of which isformed a low reflectance film. Still alternatively, the phenomenon oftotal reflection, generated by adjusting the incident angle, may also beutilized.

I claim:
 1. A laser light generating device comprising:a laser lightsource; a phase modulation means having high frequency signal generatingmeans and an electro-optical crystal, said high frequency signalgenerating means generating and outputting high frequency electricalsignals having a sole frequency component or plural frequencycomponents, said phase modulation means causing a laser beam radiated bysaid laser light source to be transmitted through said electro-opticalcrystal, to which said high frequency electrical signals are impressed,for phase-modulating said laser light; and a wavelength conversion meanshaving a non-linear optical crystal, said wavelength conversion meanscausing the laser light phase-modulated by said phase modulation meansto be transmitted through said non-linear optical crystal for convertingthe wavelength of said laser beam, wherein said laser light source is aQ-switch laser having a solid laser element.
 2. A laser light generatingdevice comprising:a laser light source; a phase modulation means havinghigh frequency signal generating means and an electro-optical crystal,said high frequency signal generating means generating and outputtinghigh frequency electrical signals having a sole frequency component orplural frequency components, said phase modulation means causing a laserbeam radiated by said laser light source to be transmitted through saidelectro-optical crystal, to which said high frequency electrical signalsare impressed, for phase-modulating said laser light; and a wavelengthconversion means having a non-linear optical crystal, said wavelengthconversion means causing the laser light phase-modulated by said phasemodulation means to be transmitted through said non-linear opticalcrystal for converting the wavelength of said laser beam, wherein saidlaser light source is a continuously oscillated laser light source.
 3. Alaser light generating device comprising:a laser light source; a phasemodulation means having high frequency signal generating means and anelectro-optical crystal, said high frequency signal generating meansgenerating and outputting high frequency electrical signals having asole frequency component or plural frequency components, said phasemodulation means causing a laser beam radiated by said laser lightsource to be transmitted through said electro-optical crystal, to whichsaid high frequency electrical signals are impressed, forphase-modulating said laser light; and a wavelength conversion meanshaving a non-linear optical crystal, said wavelength conversion meanscausing the laser light phase-modulated by said phase modulation meansto be transmitted through said non-linear optical crystal for convertingthe wavelength of said laser beam wherein said laser light source is alaser light source oscillating a laser light at a longitudinal singlemode.
 4. A laser light generating device comprising:a laser lightsource; a phase modulation means having high frequency signal generatingmeans and an electro-optical crystal, said high frequency signalgenerating means generating and outputting high frequency electricalsignals having a sole frequency component or plural frequencycomponents, said phase modulation means causing a laser beam radiated bysaid laser light source to be transmitted through said electro-opticalcrystal, to which said high frequency electrical signals are impressed,for phase-modulating said laser light; and a wavelength conversion meanshaving a non-linear optical crystal, said wavelength conversion meanscausing the laser light phase-modulated by said phase modulation meansto be transmitted through said non-linear optical crystal for convertingthe wavelength of said laser beam wherein said laser light source is alaser light source causing oscillations of a laser light at alongitudinal multiple mode.
 5. A laser light generating devicecomprising:a laser light source; a phase modulation means having highfrequency signal generating means and an electro-optical crystal, saidhigh frequency signal generating means generating and outputting highfrequency electrical signals having a sole frequency component or pluralfrequency components, said phase modulation means causing a laser beamradiated by said laser light source to be transmitted through saidelectro-optical crystal, to which said high frequency electrical signalsare impressed, for phase-modulating said laser light; and a wavelengthconversion means having a non-linear optical crystal, said wavelengthconversion means causing the laser light phase-modulated by said phasemodulation means to be transmitted through said non-linear opticalcrystal for converting the wavelength of said laser beam wherein saidelectro-optical crystal is comprised at least of potassium phosphatetitanate, potassium phosphate titanate derivatives, β-barium borate,lithium borate, lithium tantalate, dihydrogen potassium phosphate anddihydrogen ammonium phosphate.
 6. A laser light generating devicecomprising:a laser light source; a phase modulation means having highfrequency signal generating means and an electro-optical crystal, saidhigh frequency signal generating means generating and outputting highfrequency electrical signals having a sole frequency component or pluralfrequency components, said phase modulation means causing a laser beamradiated by said laser light source to be transmitted through saidelectro-optical crystal, to which said high frequency electrical signalsare impressed, for phase-modulating said laser light; and a wavelengthconversion means having a non-linear optical crystal, said wavelengthconversion means causing the laser light phase-modulated by said phasemodulation means to be transmitted through said non-linear opticalcrystal for converting the wavelength of said laser beam wherein saidphase modulation means is comprised of plural phase modulation unitsoperating at respective different frequencies.
 7. A laser lightgenerating device comprising:a laser light source; a phase modulationmeans having high frequency signal generating means and anelectro-optical crystal, said high frequency signal generating meansgenerating and outputting high frequency electrical signals having asole frequency component or plural frequency components, said phasemodulation means causing a laser beam radiated by said laser lightsource to be transmitted through said electro-optical crystal, to whichsaid high frequency electrical signals are impressed, forphase-modulating said laser light; and a wavelength conversion meanshaving a non-linear optical crystal, said wavelength conversion meanscausing the laser light phase-modulated by said phase modulation meansto be transmitted through said non-linear optical crystal for convertingthe wavelength of said laser beam wherein said phase modulation meanshas a coil and an electro-optical crystal and wherein a resonance systemformed by said coil and the electro-optical crystal has an operation ofamplifying a driving voltage impressed across the electro-opticalcrystal.
 8. A laser light generating device comprising:a laser lightsource; a phase modulation means having high frequency signal generatingmeans and an electro-optical crystal, said high frequency signalgenerating means generating and outputting high frequency electricalsignals having a sole frequency component or plural frequencycomponents, said phase modulation means causing a laser beam radiated bysaid laser light source to be transmitted through said electro-opticalcrystal, to which said high frequency electrical signals are impressed,for phase-modulating said laser light; and a wavelength conversion meanshaving a non-linear optical crystal, said wavelength conversion meanscausing the laser light phase-modulated by said phase modulation meansto be transmitted through said non-linear optical crystal for convertingthe wavelength of said laser beam wherein said phase modulation meanshas a microwave waveguide and an electro-optical crystal and wherein amicrowave resonance system formed by arraying the electro-opticalcrystal in said micro-wave waveguide has the action of amplifying thedriving voltage impressed across said electro-optical crystal.
 9. Alaser light generating device comprising:a laser light source; a phasemodulation means having high frequency signal generating means and anelectro-optical crystal, said high frequency signal generating meansgenerating and outputting high frequency electrical signals having asole frequency component or plural frequency components, said phasemodulation means causing a laser beam radiated by said laser lightsource to be transmitted through said electro-optical crystal, to whichsaid high frequency electrical signals are impressed, forphase-modulating said laser light; and a wavelength conversion meanshaving a non-linear optical crystal, said wavelength conversion meanscausing the laser light phase-modulated by said phase modulation meansto be transmitted through said non-linear optical crystal for convertingthe wavelength of said laser beam, wherein said phase modulation meansis configured so that the input fundamental wavelength laser beam isrepeatedly transmitted a plural number of times through theelectro-optical crystal.
 10. A laser light generating devicecomprising:a laser light source; a phase modulation means having highfrequency signal generating means and an electro-optical crystal, saidhigh frequency signal generating means generating and outputting highfrequency electrical signals having a sole frequency component or pluralfrequency components, said phase modulation means causing a laser beamradiated by said laser light source to be transmitted through saidelectro-optical crystal, to which said high frequency electrical signalsare impressed, for phase-modulating said laser light; and a wavelengthconversion means having a non-linear optical crystal, said wavelengthconversion means causing the laser light phase-modulated by said phasemodulation means to be transmitted through said non-linear opticalcrystal for converting the wavelength of said laser beam wherein saidphase modulation means has a groove formed on the surface of theelectro-optical crystal along an axis of transmission of the laser beamto be modulated and wherein one of the electrodes is formed on aninternal surface of said groove.
 11. A laser light generating devicecomprising:a laser light source; a phase modulation means having highfrequency signal generating means and an electro-optical crystal, saidhigh frequency signal generating means generating and outputting highfrequency electrical signals having a sole frequency component or pluralfrequency components, said phase modulation means causing a laser beamradiated by said laser light source to be transmitted through saidelectro-optical crystal, to which said high frequency electrical signalsare impressed, for phase-modulating said laser light; and a wavelengthconversion means having a non-linear optical crystal, said wavelengthconversion means causing the laser light phase-modulated by said phasemodulation means to be transmitted through said non-linear opticalcrystal for converting the wavelength of said laser beam, wherein saidelectro-optical crystal is comprised at least of potassium phosphatetitanate, potassium phosphate titanate derivatives, β-barium borate,lithium borate, lithium tantalate, dihydrogen potassium phosphate anddihydrogen ammonium phosphate.
 12. A laser beam generating devicecomprising:a laser light source; phase modulation means having highfrequency signal generating means and an electro-optical crystal, saidhigh frequency signal generating means generating and outputting highfrequency electrical signals having a sole frequency component or pluralfrequency components, said phase modulation means causing a laser lightradiated by said laser light source to be transmitted through saidelectro-optical crystal, to which said high frequency electrical signalsare impressed, for phase-modulating said laser light for generating alaser beam having four or more spectral lines, each having a spectralintensity not more than 30% of the sum total of the total spectralintensity, with the total spectral width being 500 MHz or more; andwavelength conversion means having a non-linear optical crystal, saidwavelength conversion means having wavelength conversion means forconverting the wavelength of the laser light phase-modulated by saidphase modulation means into other wavelengths.
 13. A laser beacon deviceoperating by resonant excitation of atoms in atmosphere, said laserbeacon device having a laser light source;a phase modulation meanshaving high frequency signal generating means and an electro-opticalcrystal, said high frequency signal generating means generating andoutputting high frequency electrical signals having a sole frequencycomponent or plural frequency components, said phase modulation meanscausing a laser light radiated by said laser light source to betransmitted through said electro-optical crystal, to which said highfrequency electrical signals are impressed, for phase-modulating saidlaser light; and a laser light generating means having a non-linearoptical crystal, said laser light generating means causing the laserlight phase-modulated by said phase modulation means to be transmittedthrough said non-linear optical crystal for converting the wavelength ofsaid laser beam, wherein said laser light source is a Q switch laserhaving a solid laser element.
 14. A laser beacon device operating byresonant excitation of atoms in atmosphere, said laser beacon devicehaving a laser light source;a phase modulation means having highfrequency signal generating means and an electro-optical crystal, saidhigh frequency signal generating means generating and outputting highfrequency electrical signals having a sole frequency component or pluralfrequency components, said phase modulation means causing a laser lightradiated by said laser light source to be transmitted through saidelectro-optical crystal, to which said high frequency electrical signalsare impressed, for phase-modulating said laser light; and a laser lightgenerating means having a non-linear optical crystal, said laser lightgenerating means causing the laser light phase-modulated by said phasemodulation means to be transmitted through said non-linear opticalcrystal for converting the wavelength of said laser beam, wherein saidphase modulation means has a coil and an electro-optical crystal andwherein a resonance system formed by said coil and the electro-opticalcrystal amplifies a driving voltage impressed across the electro-opticalcrystal.
 15. A laser beacon device operating by resonant excitation ofatoms in atmosphere, said laser beacon device having a laser lightsource;a phase modulation means having high frequency signal generatingmeans and an electro-optical crystal, said high frequency signalgenerating means generating and outputting high frequency electricalsignals having a sole frequency component or plural frequencycomponents, said phase modulation means causing a laser light radiatedby said laser light source to be transmitted through saidelectro-optical crystal, to which said high frequency electrical signalsare impressed, for phase-modulating said laser light; and a laser lightgenerating means having a non-linear optical crystal, said laser lightgenerating means causing the laser light phase-modulated by said phasemodulation means to be transmitted through said non-linear opticalcrystal for converting the wavelength of said laser beam, wherein saidphase modulation means has a microwave waveguide and an electro-opticalcrystal and wherein a microwave resonance system formed by locating theelectro-optical crystal in said micro-wave waveguide amplifies thedriving voltage impressed across said electrooptical crystal.
 16. Alaser beacon device operating by resonant excitation of atoms inatmosphere, said laser beacon device having a laser light source;a phasemodulation means having high frequency signal generating means and anelectro-optical crystal, said high frequency signal generating meansgenerating and outputting high frequency electrical signals having asole frequency component or plural frequency components, said phasemodulation means causing a laser light radiated by said laser lightsource to be transmitted through said electro-optical crystal, to whichsaid high frequency electrical signals are impressed, forphase-modulating said laser light; and a laser light generating meanshaving a non-linear optical crystal, said laser light generating meanscausing the laser light phase-modulated by said phase modulation meansto be transmitted through said non-linear optical crystal for convertingthe wavelength of said laser beam, wherein said phase modulation meansis configured so that the input fundamental wavelength laser beam isrepeatedly transmitted a plural number of times through theelectro-optical crystal.
 17. A laser beacon device operating by resonantexcitation of atoms in atmosphere, said laser beacon device having alaser light source;a phase modulation means having high frequency signalgenerating means and an electro-optical crystal, said high frequencysignal generating means generating and outputting high frequencyelectrical signals having a sole frequency component or plural frequencycomponents, said phase modulation means causing a laser light radiatedby said laser light source to be transmitted through saidelectro-optical crystal, to which said high frequency electrical signalsare impressed, for phase-modulating said laser light; and a laser lightgenerating means having a non-linear optical crystal, said laser lightgenerating means causing the laser light phase-modulated by said phasemodulation means to be transmitted through said non-linear opticalcrystal for converting the wavelength of said laser beam, wherein saidphase modulation means has a groove formed on the surface of theelectro-optical crystal along an axis of transmission of the laser beamto be modulated and wherein one of the electrodes is formed on aninternal surface of said groove.
 18. A laser image display device forsweeping visible laser light on a screen, said laser image displaydevice having a laser light source;a phase modulation means a havinghigh frequency signal generating means and an electro-optical crystal,said high frequency signal generating means generating and outputtinghigh frequency electrical signals having a sole frequency component orplural frequency components, said phase modulation means causing a laserlight radiated by said laser light source to be transmitted through saidelectro-optical crystal, to which said high frequency electrical signalsare impressed, for phase-modulating said laser light; and a laser lightgenerating means having a non-linear optical crystal, said laser lightgenerating means causing the laser light phase-modulated by said phasemodulation means to be transmitted through said non-linear opticalcrystal for converting the wavelength of said laser beam, wherein saidlaser light source is a Q switch laser having a solid laser element. 19.A laser image display device for sweeping visible laser light on ascreen, said laser image display device having a laser light source;aphase modulation means a having high frequency signal generating meansand an electro-optical crystal, said high frequency signal generatingmeans generating and outputting high frequency electrical signals havinga sole frequency component or plural frequency components, said phasemodulation means causing a laser light radiated by said laser lightsource to be transmitted through said electro-optical crystal, to whichsaid high frequency electrical signals are impressed, forphase-modulating said laser light; and a laser light generating meanshaving a non-linear optical crystal, said laser light generating meanscausing the laser light phase-modulated by said phase modulation meansto be transmitted through said non-linear optical crystal for convertingthe wavelength of said laser beam, wherein said phase modulation meanshas a coil and an electro-optical crystal and wherein a resonance systemformed by said coil and the electro-optical crystal amplifies a drivingvoltage impressed across the electro-optical crystal.
 20. A laser imagedisplay device for sweeping visible laser light on a screen, said laserimage display device having a laser light source;a phase modulationmeans a having high frequency signal generating means and anelectro-optical crystal, said high frequency signal generating meansgenerating and outputting high frequency electrical signals having asole frequency component or plural frequency components, said phasemodulation means causing a laser light radiated by said laser lightsource to be transmitted through said electro-optical crystal, to whichsaid high frequency electrical signals are impressed, forphase-modulating said laser light; and a laser light generating meanshaving a non-linear optical crystal, said laser light generating meanscausing the laser light phase-modulated by said phase modulation meansto be transmitted through said non-linear optical crystal for convertingthe wavelength of said laser beam, wherein said phase modulation meanshas a microwave waveguide and an electro-optical crystal and wherein amicrowave resonance system formed by locating the electro-opticalcrystal in said micro-wave waveguide amplifies the driving voltageimpressed across said electro-optical crystal.
 21. A laser image displaydevice for sweeping visible laser light on a screen, said laser imagedisplay device having a laser light source;a phase modulation means ahaving high frequency signal generating means and an electro-opticalcrystal, said high frequency signal generating means generating andoutputting high frequency electrical signals having a sole frequencycomponent or plural frequency components, said phase modulation meanscausing a laser light radiated by said laser light source to betransmitted through said electro-optical crystal, to which said highfrequency electrical signals are impressed, for phase-modulating saidlaser light; and a laser light generating means having a non-linearoptical crystal, said laser light generating means causing the laserlight phase-modulated by said phase modulation means to be transmittedthrough said non-linear optical crystal for converting the wavelength ofsaid laser beam, wherein said phase modulation means is configured sothat the input fundamental wavelength laser beam is repeatedlytransmitted a plural number of times through the electro-opticalcrystal.
 22. A laser image display device for sweeping visible laserlight on a screen, said laser image display device having a laser lightsource;a phase modulation means a having high frequency signalgenerating means and an electro-optical crystal, said high frequencysignal generating means generating and outputting high frequencyelectrical signals having a sole frequency component or plural frequencycomponents, said phase modulation means causing a laser light radiatedby said laser light source to be transmitted through saidelectro-optical crystal, to which said high frequency electrical signalsare impressed, for phase-modulating said laser light; and a laser lightgenerating means having a non-linear optical crystal, said laser lightgenerating means causing the laser light phase-modulated by said phasemodulation means to be transmitted through said non-linear opticalcrystal for converting the wavelength of said laser beam, wherein saidphase modulation means has a groove formed on the surface of theelectro-optical crystal along an axis of transmission of the laser beamto be modulated and wherein one of the electrodes is formed on aninternal surface of said groove.